OCR Gateway Additional Science B3, B4, P3, P4, C3 and C4

An insulating material can become electrically charged if it is rubbed with another material. This charge is static; meaning that it does not move. The static charge involves the transfer of electrons from one material to the other - leaving one with a positive charge and the other with a negative charge.

When static electricity is generated, the statically charged object will attract smaller objects - such as a dusting brush attracting dust.

An object can be discharged by earthing it. When an object discharges, electrons are transferred from the charged object to the earth. If you become charged, earthing can result in you getting an electrostatic shock.

Two examples of charges becoming earthed:

When a person is in a car for a long period of time then they can become charged due to them rubbing against the seat, creating friction. When they step out of the car and touch the ground discharge can occur - resulting in an electrostatic shock.

A person can also become charged when friction builds up between them and the carpet on which they're walking. When they touch a water pipe, e.g. a radiator, the charge becomes earthed which again can result in an electrostatic shock.

One of the main problems with static electricity occurs in places such as flour mills and petrochemical factories. These places have atmospheres that contain a high amount of oxygen and this can ignite if there is a discharge of static electricity (i.e. a spark).

Two insulating materials (such as Perspex rods) with the same charge will repel each other whilst two insulating materials with different charges will attract each other.

Electric charge (static) builds up when electrons are rubbed off one material onto another. The material that receives the electrons becomes charged (due to excess electrons) while the material giving up the electrons becomes positively charged (due to a loss of electrons).

The chance of receiving an electric shock can be reduced by:

Ensuring appliances are correctly earthed;

Using insulation mats effectively;

and wearing shoes with insulating soles.

Lorries that contain inflammable gases, liquids or powders need to be earthed before unloading, as friction can cause a build-up of charge. This is to stop a spark which could possibly ignite the flammable substance.

Anti-static sprays, liquids and cloths help to reduce the problems of static electricity bypreventing the transfer of charge from one insulator to the other - with no build-up of charge, there can be no discharge.

Photocopiers

An image of the page to be copied is projected onto an electrically charged plate, which usually has a positive charge. Light causes charge to leak away, leaving an electrostatic impression of the page. The charged impression on the plate attracts tiny specks of black powder, which are then transferred from the plate to paper. Heat is then used to fix the final image of the paper.

Laser Printers

A printer cartridge contains a rotating drum

Most chemical reactions take place in the cytoplasm (which may also contain mitochondria which is where most energy is released in respiration).The cell membrane controls movement into and out of the cell.The nucleus contains the genetic information and controls what the cell does. It has a membrane extending from it, onto which ribosomes are attached.Protein synthesis (making) takes place in the ribosomes and the DNA controls the process. Proteins are made up of chains of amino acids are they are essential for the growth and repair of cells. We make some amino acids (non-essential) but also get others from our food (essential). Each protein has a different function. The liver produces some proteins in transamination - a process in which the structure of some amino acids are changed.When a cell copies itself it first unzips. Then the new bases pair up with the exposed bases on each strand. Afterwards, an enzyme bonds the new bases together to form complementary strands helping to from two identical pieces of DNA.The nucleus of a cell contains a complete set of genetic instructions. The information is carried by genes on chromosomes, which are made from DNA. A DNA molecule is made of two strands coiled around each other in a double helix. The gentic instructions are in the form of a chemical code made up of four bases. These bases bond together in pairs, forming the cross-links which hold the coil together. There are four bases in total - A (adenine), C (cytosine), G (guanine) and T (thymine). A always pairs with T and C always bonds with G. Before a cell divides it replicates its DNA to make sure the new cells contain a complete set of genetic information. Each person’s DNA is unique and can be used

to identify them, one example of this is DNA fingerprinting. There are four stages to fingerprinting and these are: Isolation - the DNA is extracted from blood, hair follicles or semen

Fragmentation - the DNA is cut into fragments using enzymes called restriction enzymes

Separation - the DNA sections are separated using a technique called electrophoresis.

Comparison - the DNA fingerprint is analysed by comparing it with a reference sample collected from the crime scene.